1. Field of the Invention
The present invention relates to an optical waveguide which includes a core for transmitting an optical signal, and a mirror for reflecting the optical signal, a method of manufacturing the optical waveguide, and an optical transmission device which includes the optical waveguide.
2. Description of the Related Art
Heretofore, in multifarious electronic circuits, optical/electrical hybrid circuits in each of which part of the corresponding circuit is changed from electrical wiring of copper or the like into optical wiring based on an optical waveguide have been employed with the raised speed (heightened frequency) of signal transfer, and various relevant techniques have been proposed (refer to JP-A-2-118607 and JP-A-8-46292).
In general, in an optical/electrical hybrid circuit, a light emitting element such as vertical cavity surface emitting laser (VCSEL) and a light receiving element such as photodiode (PD) are mounted on a circuit board, and light (an optical signal) emitted from the light emitting element enters the light receiving element through an optical waveguide.
In such an optical/electrical hybrid circuit, there is adopted a method wherein the light vertically emitted from the light emitting element toward the circuit board is reflected 90 degrees so as to become horizontal to the circuit board, by a mirror portion whose light travel direction has an angle of 45 degrees, whereby the light propagates within the optical waveguide.
Here, a method of manufacturing an optical waveguide 101 according to a prior-art example will be explained.
First, as shown in FIG. 12, the step of successively stacking a first clad layer 110, a core 112 and a second clad layer 114 on a dummy board 102 is performed by well-known techniques. Any of the layers is formed using a resin material which is capable of transmitting an optical signal (for example, silicone), and a material of relatively high refractive index is used for the core 112, whereas a material of relatively low refractive index is used for the first clad layer 110 and the second clad layer 114.
Subsequently, as shown in FIG. 13, the step of forming V-shaped grooves 103 and 104 which divide the core 112 is performed by a dicer (dicing device) from the side of the second clad layer 114. On this occasion, the grooves 103 and 104 are formed so that an angle α1 which is defined between a slant surface 103a constituting the groove 103 and the upper surface 114a of the second clad layer 114, and an angle α2 which is defined between a slant surface 104a constituting the groove 104 and the upper surface 114a of the second clad layer 114 may become 45 degrees, respectively.
Incidentally, another example of the step of forming the grooves 103 and 104 may be performed such that a gray mask is provided on the upper surface 114a of the second clad layer 114, and that the slant surfaces 103a and 104a which define the angles α1 and α2 of 45 degrees, respectively, similarly to the above are formed by a known photolithographic process (not shown).
Subsequently, as shown in FIG. 14, the step of forming metallic reflection films 116 on the slant surface 103a of the groove 103 and the slant surface 104a of the groove 104 is performed. Thus, a mirror 105 is formed on the slant surface 103a, and a mirror 106 on the slant surface 104a. 
Subsequently, as shown in FIG. 15, the step of transferring the resulting structure on the dummy board 102, from the dummy board 102 onto a formal board 107 is performed. The manufacture of the optical waveguide 101 is carried out including the steps explained above. Incidentally, arrows in FIG. 15 indicate a path along which the optical signal passes.
In the above manufacturing method, however, it has been difficult to highly precisely and minutely form the slant surfaces 103a and 104a on which the mirrors 105 and 106 are respectively disposed. More specifically, when the slant surfaces 103a and 104a are formed by the dicing, the surface roughness thereof becomes large. This can lead to the problem that light propagation losses appear in the mirrors 105 and 106 which are fabricated by forming the metallic reflection films 116 on the surfaces 103a and 104a. On the other hand, when the slant surfaces 103a and 104a are formed by the photolithographic process, it is difficult to form the angles α1 and α2 accurately at 45[°] with respect to the upper surface 114a of the second clad layer 114. This can lead to the problem that light propagation losses appear in the mirrors 105 and 106 which are fabricated by forming the metallic reflection films 116 on the surfaces 103a and 104a. 
Besides, in case of performing the steps of forming the structure on the dummy board 102 and thereafter transferring the structure onto the formal board 107 as stated above, there has been the problem that the positioning precision between the optical waveguide 101 and the formal board 107 worsens.
Besides, the optical waveguide portion is protuberant beyond the mounting surface 107a of the formal board 107 in correspondence with its thickness. This has led to the problem that a mounting pad for mounting electronic components such as a light emitting element and a light receiving element must be formed having a raised level.